I'm getting about 10 dB of gain reduction with this routing. If I mute the compressor, the gain jumps up 10 dB. I'm thinking it must be comb filtering, but what do I know?

I've tried the same setup with a second compressor (Fabfilter compressor) I get about 5 dB of gain reduction. But if I'm doing this right, and if the plugins are compensating for latency, then I should be getting no gain reduction at all, right? (In fact, I'm thinking that I should be getting a modest gain increase because of the extra channel (the compressor bus) being added to the dry signal. No?)

Compressors reduce gain--that's how they work and why there's an output gain or "make up gain" control on them. Although it's easy to think of them "bringing up the quiet bits what they in fact do is reduce the level on all parts of the signal above the threshold you've set by the amount you've set. You then add the output gain to whatever amount is needed to give you the signal at whatever level you want but with reduced dynamic range.

Comb filtering generally shows up, not as reduced gain overall but rather as a very strange frequency response with frequencies alternately boosted and cut. It's a pretty distinctive sound when you hear it.

If you're using them in parallel and there's even the slightest processing delay difference (although there shouldn't be) then comb filtering is a possibility, because you'll have the equivalent of a 'flange', only fixed in time.

Stupid. Of course it has make up gain. That's not the issue. Okay, I really don't get it. Why would sending the dry track to a buss with the Izotope compressor on it drop the level of the track so significantly?

Because the Izotope signal is in anti phase with the direct one, thus causing partial cancellation of the audio signal. Because the Izotope signal is compressed the level is not equal to the direct one all the time, hence not complete cancellation.

"Because the Izotope signal is in anti phase with the direct one, thus causing partial cancellation of the audio signal."

Hm? Anti phase? What's that? And if it's in 'anti phase', why is there not complete cancellation? And why does the Izotope compressor do this anyway?

He means that it's signal inverted - actually it hasn't got anything to do with phase at all. No I don't know why, and I haven't tried it - although it seems somewhat unlikely, I must say.

"Because the Izotope signal is compressed the level is not equal to the direct one all the time, hence not complete cancellation."

Hm?

That's easier to answer. Compression inevitably means that the signals at different amplitudes are going to be at different levels when it comes out of the compressor. If they weren't it wouldn't be compressed... so cancellation would only be complete when the inverted signal was at exactly the same level as the original non-inverted signal. Where there was a level change so that the signals weren't at the same level, then cancellation couldn't be complete.

Mind you, I'm not convinced (without hearing it) that this has anything to do with what's going on at all...